1. Field of the Invention
The invention is directed to a sensor system and a method for providing a higher degree of specificity and sensitivity in cantilever sensing. More specifically, the invention relates to the binding of substrates to target specific receptors to improve detection specificity and sensitivity in cantilever sensors.
2. Description of the Related Technology
The National Center for Health Statistics reports that cancer is the second leading cause of death in the United States, accounting for 22.7% of deaths in 2003. Many of these deaths may have been prevented by early detection and diagnosis, which offers the most chance of successfully fighting cancer. In an attempt to advance early detection technologies, scientists have been trying to develop biosensors capable of identifying minute amounts of specific proteins in a sample that contains multiple protein species to create a sensitive blood test for detecting cancer markers. The presence of minute amounts of cancer marker proteins or debris shed by cancer cells present in patients' blood, however, is typically overshadowed by the large amount of proteins normally contained in serum samples. The lack of sufficient detection specificity and sensitivity, therefore, is a critical and currently unresolved issue hindering the development of viable diagnostic tools based on protein detection.
One of the most common commercially available sensor systems for biosensing applications, the quartz crystal microbalance (QCM), uses thickness-mode resonance detection. The detection sensitivity of a QCM is related to the resonance frequency and the thickness of the quartz membrane. A resonance frequency of about 5 MHz, corresponding to a quartz membrane thickness of 330 μm, enables a minimum detectable mass density of about 10−9 g/cm2. Sensitivity is therefore generally limited to a range of about 10−8 g/Hz. Although convenient and economical, the QCM has a relatively low detection sensitivity in the ng/ml range. By contrast, protein detection for diagnostic purposes generally requires a sensitivity in the range of pg/ml to fg/ml.
Another popular sensor system, ELISA, for example, has a detection sensitivity of 0.01-50 ng/ml for proteins in solution, depending on the affinity of antibodies. Detection using the ELISA system, however, is time intensive and requires labeling. Recently, H. Zhang, et al., “A Sensitive and High-Throughput Assay to Detect Low-Abundance Proteins in Serum,” Nature Medicine 12 (4) 473-477 (2006) showed that ELISA detection can be enhanced 105 fold (5 pg/ml) by replacing enzymes with RNA polymerase in HER-2 detection. Even with this improvement, processing speed and expense remain a significant concern.
Some methods for increasing the sensitivity of current sensors are known. One example of this is found in J.-W. Park, S. Kurosawa, H. Aizawa Y. Goda, M. Takai and K. Ishihara, “Piezoelectric Immunosensor for Bisphenol A Based on Signal Enhancing Step With 2-methacrolyloxyethyl Phosphorylcholine Polymeric Nanoparticle,” Analyst, 131, 155-162 (2006).
Current state of the art sensors, therefore, are incapable of both real-time label free detection and highly sensitive detection. This shortcoming is particularly problematic for applications involving biological samples, which typically have a high protein concentration that creates noise, interfering with detection sensitivity. For example, the ability to detect minute amounts of a target protein or DNA in sera is hindered by the background serum protein concentrations in the test sample. The large amount of protein in sera significantly dilutes biomarker concentration, which in turn reduces detection sensitivity.
Current sensors also lack an independent means for verifying detection results. An independent verification means would efficiently minimize false-positive and false-negative results, which is particular critical to the field of medical diagnostics and cancer detection. Therefore, there exists a need to develop a sensor system that enables highly sensitive label free real time detection as well as verification of the detection results.